New Otago University research has found not all unwanted food is able to be re-purposed in a sustainable way

The Public Health Communication Centre has looked into how up-cycled food is environmentally, socially and economically sustainable.

Author Meg Thorsen says it shows environmental benefits rely on factors such as the type of waste being re-purposed and the energy needed for processing.

She says up-cycling brewer's spent grain into food products could have a higher carbon footprint than using it as animal feed - unless appropriate processes are used.

Thorsen says it shows there's a need for policies to balance the outcomes.